It is often necessary to control the mass airflow through the compressor of a high performance axial flow gas turbine engine in order to avoid characteristic unstable operation of the compressor, particularly during engine acceleration. Fundamentally, such control may be exercised by either of two-conventional ways: by bleeding or venting compressor stages to a suitable relatively lower pressure drain source; or by varying the angle of the stator vanes to thereby vary the effective flow area of the compressor inlet to increase or decrease the mass airflow to the compressor. The latter control may be exercised by using one or more stages of the compressor stator which have a mounting angle relative to the direction of airflow within the engine that can be varied with changing conditions. The variable vane stators may also be used in conjunction with engine overboard air bleed as a means to avoiding unstable compressor operation during certain phases of an engine's operating regime. The control in the form of a bleed valve serves to facilitate engine starting and also serves to prevent compressor surge at low speed by ducting the compressor air overboard during low thrust operation. During high thrust operation, the bleed valve should be closed. When the bleed valve is open, it increases airflow to the upstream portion of the compressor and reduces the pressure on the bottleneck downstream.
In those applications where variable vane stators are used in conjunction with engine overboard air bleed valves, it has heretofore been known to provide two separate and distinct actuators for actuating the bleed valve and the variable geometry vanes with a suitable control system for coordinating the operation of the two actuators. As an example, reference is made to U.S. Pat. No. 3,172,259 of North, Jr. entitled "Variable Geometry Control for Gas Turbine Engines" which issued on Mar. 9, 1965 and is assigned to the assignee of the subject application. In U.S. Pat. No. 3,172,259 a plurality of servo pistons and a control system are employed for actuating one or more of the compressor interstage air bleeds, compressor discharge air bleeds, compressor variable stator blades, variable turbine exhaust nozzle areas, variable angle of the turbine stator blades, compressor inlet flow fences or restrictions, and turbine by-pass or bleed valves for improving engine performance under changing operating conditions.
Along the same lines are the disclosures in U.S. Pat. No. 3,973,391 to Reed et al, U.S. Pat. No. 2,616,663 to Alm, and U.S. Pat. No. 3,873,230 to Norris et al. The prior art systems which use individual actuators for each functional requirement have resulted in certain mounting problems of the actuators, excess fuel lines, complicated control connections, and vulnerability of the actuators to possible damage due to ground fire when the engine is employed in a combat aircraft. On the other hand, the employment of a single actuator for controlling the variable vane geometry as well as the bleed valve of a gas turbine compressor is complicated by the requirement for converting the linear motion from a single actuator to multiplane intermittent rotational motion by virtue of the different positions of the bleed valve and the variable vane actuating linkages about the periphery of the casing of the engine compressor. In addition, the use of a single actuator for negotiating the multiplane relationship of the bleed valve and the variable vanes could result in a linkage arrangement which has less than optimum design characteristics, and which could give rise to side loading on the linkage components thereof or accumulative tolerances or adjustments.
Accordingly, it is an object of the subject invention to provide a unique actuating system arranged such that a single actuator is operatively associated with the actuating linkage extending to the bleed valve and the actuating linkage extending to the variable vane actuating mechanism of a compressor of a gas turbine engine, and is operative to coordinate the intermittent operation of said bleed valve and the variable vanes during the entire operating regime of the compressor.
It is another object of the subject invention to provide an actuating system including a single actuator which is disposed in a position relative to the compressor casing so as to be less vulnerable to damage due to ground fire when employed in military aircraft.
Still another object of the subject invention is to provide a new and improved actuating system which is reliable, requires only a single actuator, and which provides multiplane intermittent rotational motion to the air bleed valve and the variable vane actuating linkages of a compressor.
A further object of the invention is to provide an actuator linkage arrangement which has optimum design characteristics with minimum side loading of the linkages.